Polyester/poly(methyl methacrylate) articles and methods to make them

ABSTRACT

A polymeric composition comprised of poly(methylmethacrylate) (PMMA) and polylactic acid (PLA) having a surface charge potential of at least about 50 volts in the absence of any other charge enhancing component may be made by melt blending PMMA and PLA, extruding the melt blend through a die and cooling at a rate through Tg of the PLA of at 10° C./min to 1000° C./second. The polymeric composition may be made by melt blowing into a nonwoven fabric. The nonwoven fabric may be charged to a surface potential of at least about 50 electron volts. Such filters may have greater than 95% efficiency at a pressure drop of less than 2 mm Hg even after being exposed to high temperatures (~70° C.) for an hour or more.

FIELD OF THE DISCLOSURE

The present disclosure relates to articles comprised of polyesters/poly(methyl methacrylate) polymer blends. In particular, the inventionrelates to nonwoven filters comprised of polylactic acid/poly (methylmethacrylate) polymer blends.

BACKGROUND

Conventional filters have been made from many polymers such aspolypropylene, polyethylene, polyester, polyamide, polyvinyl chloride,and polymethyl methylacrylate. The filters typically are made by a meltblown process forming a nonwoven fabric having substantially nonuniformlength and diameter fibers with globules of pooled polymer causing aless than desired pressure drop across the filter for many applications.

Conventional filters lack an electrostatic charge and filter byimpingement, impactions and diffusion. Electrostatic charge has beenapplied to improve the filtration performance of conventional filters.The charge causes the filter media to electrostatically attract smallair borne particles and enhance the attachment of the particles to themedia surface, thus improving efficiency of the filter.

Unfortunately, the electrostatic charge is known to dissipate with timeand with the application of heat lowering the performance of thefilters. To remedy the problem, charge enhancing additives such as fattyacid stearates or amides have been added to the polymers used to makethe filters (see U.S. Pat. Publ. US20060079145). These, however, mayaffect the processing and fibers formed as well as potentially causeallergic reactions for applications that contact skin.

Accordingly, it would be desirable to provide a polymer and filter thatovercomes one or more problems of the filtration art such as describedabove and, in particular, to filter applications that may come incontact with human skin (e.g., masks).

SUMMARY OF THE INVENTION

An aspect of the invention is the discovery that particular blends ofPLA/PMMA may be melt blended, sheared and rapidly cooled to formarticles such as nonwoven fabrics that display excellent ability to becharged enabling excellent filtration of particulates as well as retainthe surface charge and maintain filtration efficiency. Thus, this aspectof the invention is a polymeric composition of poly(methylmethacrylate)and polylactic acid having a surface charge potential of at least about50 electron volts in the absence of any other charge enhancing componentor any other additive. In particular, the composition surprisinglyretains at least 90% of said charge even when exposed to high heat (70°C.) for an hour to 24 hours.

Another aspect of the invention is that PMMA/PLA blends may be formedinto particularly useful non-woven fabrics useful for filtrationapplications. The melt blown fabrics with long uniform diameter fiberswithout the defects observed in commercial polypropylene melt blownfabrics made for similar uses. This aspect is a method of producing anon-woven material comprising: providing a feed mixture comprisingpolymethyl methacrylate and polylactic acid to a melt blown extrusiondevice; liquefying the feed mixture in the extrusion device to form aliquified feed mixture; blowing the liquified feed mixture through atleast one nozzle to form a spray; depositing the spray on a surface; e)reducing a temperature of the spray such that the spray solidifies onthe surface to form a sheet of non-woven material.

Another aspect of the invention is a method of forming a polymericcomposition comprising:(i) melting and blending polylactic acid andpoly(methyl methacrylate) to form a melt blend, (ii) extruding the meltblend through a die to form a shaped article and (iii) cooling theshaped article at a cooling rate through Tg of the polylactic acid suchthat a microstructure is formed that may be charged to surface potentialof at least 50, 100, 200, 500 or 1000 electron volts. This aspect may beperformed in a manner using the method in the previous aspect.

A further aspect of the invention is a filter comprising at least onelayer of a nonwoven fabric comprised of fibers ofpoly(methylmethacrylate) and polylactic acid that have a diameter ofabout 0.1 micrometer to 15 micrometers. In a particular embodiment thefilter fibers are from about 0.5 or 1 micrometer to 15 micrometers withan average diameter of about 2 to 6 micrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed non-limiting embodiments are discussed in relation to thedrawings appended hereto and forming part hereof, wherein like numeralsindicate like elements, and in which:

FIG. 1 shows an exemplary melt blown extrusion system.

FIG. 2 shows a face mask apparatus.

FIG. 3 shows a scanning electron microscope image of a PMMA/PLA filterof this invention.

FIG. 4 shows a scanning electron microscope image of a PMMA/PLA filterof this invention.

FIG. 5 shows a scanning electron microscope image of a PMMA/PLA filterof this invention.

FIG. 6 shows a scanning electron microscope image of a PMMA/PLA filterof this invention.

FIG. 7 shows a scanning electron microscope image of a PMMA/PLA filterof this invention.

FIG. 8 shows a scanning electron microscope image of a PMMA/PLA filterof this invention.

FIG. 9 shows a scanning electron microscope image of a PMMA/PLA filterof this invention.

FIG. 10 shows a scanning electron micrograph of a commercialpolypropylene melt blown filter.

DETAILED DESCRIPTION

Non-woven materials useful as filters or masks may be produced fromnumerous materials, which may include one or more of the following:polymers, polymer blends, shape memory materials, shape memory polymers,and shape memory polymer blends.

Nonwoven fabrics useful to make filters and masks may be made by meltblowing. A blended polymer composition comprising polymethylmethacrylate (PMMA) and polylactic acid (PLA) may be melt blown. Theblend of PMMA (30% w/w) and PLA (70% w/w) is fed to a melt blownextrusion machine, such as the one shown in FIG. 1 . FIG. 1 shows anexemplary melt blown extrusion system 800. An extruder 801 receives aPMMA/PLA feed material (or other material) from hopper 802, liquefiesthe feed material, and blows/sprays the liquefied feed material throughnozzle openings 803. High velocity air (not shown) may be used to assistin blowing/spraying the liquefied feed material through nozzle openings803. The blown material 804 travels through air and is deposited on arotating cylindrical surface 805, which is shown to be rotating in thedirection shown by the arrow 805 a. When the blown material 804 isdeposited onto the rotating cylindrical surface 805 at location 806, itcools and becomes solid. A solid sheet/film 807 is pulled by a secondrotating cylinder 808 that is rotating in the direction shown by thearrow 808 a. Newly added sheet/film material is added at 809 onto aspool of melt blown polymer 810.

The PMMA/PLA blend is then extruded and blown in a system such as thesystem in FIG. 1 to form a non-woven fabric sheet that can be used inface masks.

The polymeric composition typically is comprised of about 15% to about85% PMMA with the balance being PLA. Desirably the composition iscomprised of at most about 50% of PMMA. The PLA may be any form of PLAsuch as those formed using L-lactide, D-lactide, or combination thereof.Desirably, the amount of L-lactide is at least 50%, 60%, 70%, 80% or 90%to 98% or 100% (100% may include trace amounts of D-lactide) by weightof the monomer used to make the PLA. Other polyesters such as thoseknown in the art that readily form crystalline or semi-crystallinepolymers may also be used.

The PLA or polyester may have any Mw to realize a blend with the PMMAthat has the desired melt flow rate. Typically, the PLA has a weightaverage molecular weight (Mw) of about 10 kDa to 500 kDa. The melt flowrate of the PLA or polyester may be any useful to form the articlesherein such as the nonwoven fabrics. Typically, the MFR is one whencombined with the PMMA realizes the desired fiber diameters and lengthsas described herein. Generally, the MFRs may be from 25, 50, 60 or 70 to90, 100, 125 or 150 grams (210° C./10 min, 2.16 kg). Examples ofsuitable PLAs are available under the tradename INGEO Biopolymer 625Fand 3260HP from NatureWorks LLC and LUMINY L105 from Total Corbion PLA.

The PMMA may be any suitable PMMA such as those known in the art and mayhave a Mw that varies over a wide range such as from 10 kDa to 3 MDa solong as the PMMA exhibits rheological behavior allowing for the rapidshear and quenching while still realizing the desired degree ofcrystallinity and charge capability of the PLA/PMMA blend. The PMMA maybe made with a small percentage (e.g., less than about 5%, 2%, or 1%) ofcomonomers such as those commonly used in the art (e.g., methylacrylate, butyl acrylate and the like) to improve one or more propertiessuch as impact strength or heat stability. The melt flow rate of thePMMA may be any useful MFR to form the articles herein such as thenonwoven fabrics. Typically, the MFR of the PMMA is one, when combinedwith the PLA, that realizes the desired fiber diameters and lengths asdescribed herein. Generally, the MFR of the PMMA may be from 1, 2, 5, or10 to 100, 50, 40 or 30 grams (230° C./10 min, 3.8 kg). Examples ofPMMAs that may be useful include those available under the tradenamedesignation CA41 from PLASKOLITE and those available under the tradenamePLEXIGLAS VM, VS and VSUVT from Arkema.

It has been discovered, surprisingly, that the polymeric compositionthat is formed under sufficient shear and cooling through thecrystallization melt temperature, in the absence of charge enhancingadditives, allows for the charging of the composition to high surfacepotentials. This is believed, without being limiting in any way, to bedue to the microstructure that is formed which may be comprised ofsmaller crystalline grains with greater defects and grain boundariesthan previous compositions, for example, of melt blown nonwoven fibers(exposed to adequate shear and cooling). The surface charge of thepolymeric composition may be at least 50, 100, 200, 500 or even 1000electron volts. The surface charge may be measured by commerciallyavailable devices such as described in US5401446, col 8, lines 6 to 21.

Surprisingly, the PLA crystallizes when blended with amorphous (PMMA)even when sheared at high rates and rapidly cooled through the meltcrystallization temperature of the PLA or polyester. The degree ofcrystallinity by volume is from the amount of PLA that crystallizesduring the method described herein. Typically, the amount of PLA thatcrystallizes is from about 1%, 2%, 5% or 10% to about 50%, 40% or 30%.Thus, the degree of crystallinity of the blend when 50% of the PLAcrystallizes in a 70% PLA/30% PMMA blend would give about 35% degree ofcrystallinity for the blend.

The charging of the surface may be accomplished by any known method ofcharging filters and the like such as described in U.S. Pat. and Pat.Publ.: US2740184; US4215682; US4375718; US4588537; US4592815; US4904174;5122048; US5401446; and US2006/0079145. The charging voltage may be anyvoltage useful to impart the charge to the filter. Illustratively, thecharging voltage may be from about 2 kV to 50 kV.

In an embodiment, the article is a filter comprising at least one layerof a nonwoven fabric comprised of fibers of poly(methylmethacrylate) andpolylactic acid that have a diameter of about 1 micrometer to 15micrometers and average diameter of about 2 to 6 micrometers. The filterdesirably has at least about 90%, 95%, 99% or more by number of thefibers being longer than 500 micrometers, 1 millimeter or 2 millimeters.The filter nonwoven fabric surprisingly may have a low fabric weight andeven when a higher fabric weight is used the filter may have a lowpressure drop. The fabric weight may be for example 25, 50, or 100 to300 or 200 grams/m². The filter typically has pressure drop of at mostabout 5 mm Hg (666 Pa) 4 mm Hg (533 Pa), 2 mm Hg (266 Pa) or 1 mm Hg(133 Pa) even, for example, when the fabric weight is greater than 50 to300 grams/m² or multiple layers are used (layer being a separatediscreet fabric sheet for example). One or more layers may be used tomake the filter. The filter typically also may have an efficiency of atleast 95% or 98%. The filter because of the microstructure as describedherein retains the surface charge potential even when heated. Forexample, the surface potential, typically decays at most about 10% afterexposure to 70° C. for one hour with essentially no loss in efficiencyor less than 5%, 3%, 2%, or 1% loss in efficiency after such heating.

In the method of forming a fibrous article of the polymeric composition(e.g., melt blowing process) it is desirable for the PMMA/PLA blend tohave a viscosity at 200° C. that allows for the production of fibershaving a diameter of 1 to 15 micrometers with an average diameter ofabout 2 or 3 to 8, 7 or 6 micrometers. Typically, the MFR of the polymerblend of PLA and PMMA is 20 or 30 to 90 or 80 grams (210° C./10 min,2.16 kg). The diameter may be determined by measuring the diameter of anumber of fibers (~100) on one or more micrographs of the nonwovenfabric utilizing known image analysis techniques or by hand.

It has been discovered that ultra-high efficiency, the fabric, typicallynon-woven, may be comprised of fiber diameters all less than 1micrometer to about 0.05 micrometer, with the lengths as describedabove. For both this embodiment and the embodiment described in theprior paragraph, the diameter along the length of the fibers displaysurprisingly good uniformity, which is readily apparent in FIG. 5 .Typically the fiber has a diameter that has standard deviation from themean of about 20%, 10% or even 5%, which may be determined by measuringabout 50 fibers at 5 to 10 equidistant points from one end to the otherend along the fiber length in a 100X micrograph and may utilize knownimage analysis techniques.

When forming a shaped article such as a filter element, the PLA/PMMA aremelt blended and extruded through a die, typically forming a fibroussheet or fabric, which may be woven or nonwoven. The shear rate throughthe die should be sufficient to realize the desired microstructure andsurface charging potential. Typically, the shear rate is at least 10,100, 250, 500 or 750 1/s to 5000, 4000 or 3500 1/s. Likewise, coolingrate through crystallization melt temperature of the polylactic acid orpolyester (typically from about 140° C. to 180° C.) is sufficientlyrapid such that the desired microstructure is formed allowing for thesurface charge potential to be realized upon charging of the filterelement formed by the process. Typically, the cooling rate is 10, 50,100, 1000° C./min to 5000° C./sec, 4000° C./sec or 3500° C./sec, whichis typically from about the melt temperature of the polymer blend toabout 100° C.

Illustratively, when forming a melt blown nonwoven fabric, the melt pumptemperature is above the melt point of the PLA/PMMA polymers to meltblend them and typically is from about 175 to 250° C. The dietemperature is generally slightly higher than the melt pump temperature(i.e., 10% or 20% higher). For example, if the melt pump temperature is200° C. then the die temperature may be from about 220 to 250° C. Toensure a desired cooling rate to realize the desired microstructure theair temperature from the exiting of the die to impingment on thecollection rolling is typically set to at least about 1.3, 1.5, 1.8 oreven 1.9 times greater than the melt pump temperature or dietemperature, with it being understood that the air cools somewhat uponbeing blown from the blowing orifice until impingement upon the fiberexiting the die.

Surprisingly, the non-woven fabric comprised of PMMA/PLA) may have anefficiency of 98-99% with only a 2 mm Hg (millimeters of mercury)pressure drop, having the fiber structure described here, which may bemade by the melt blown process described herein. Filtering efficiencyand pressure drop were measured using a TSI 8130A instrument. An airflow rate of 32 L/min was used for measurements with a salt (NaCl)diameter of 0.1 µm or 0.2 µm. The fabric filter contained 30% w/w PMMAand 70% w/w PLA. The filter material was electrically charged and had acircular shape with a 111 mm diameter. The filter material had a densityof 100 grams per square meter (gsm). The PMMA/PLA sheet wasthermoformable and exhibited shape memory. That is, the PMMA/PLA sheetcan be placed over a fixture and heated to an elevated temperature inorder to mold the PMMA/PLA sheet into a desired shape. It may bedesirable to heat the PMMA/PLA sheet to a temperature that isapproximately equal to, equal to, or above a transition temperature. Thetransition temperature (to heat to) may be the transition temperature ofa polymer that is present in the polymer blend. The transitiontemperature (to heat to) may be the transition temperature of thepolymer with the lowest transition temperature (compared to the otherpolymers in the polymer blend).

Advantageously, thermoforming a PMMA/PLA polymer blend results in apredefined shape without deforming the fibers. The amount of PMMA in aPMMA/PLA blend may be from 15 to 85% w/w to produce a PMMA/PLA blendthat exhibits shape memory properties. Likewise, the amount of PLA in aPMMA/PLA blend may be from 15 to 85% w/w to produce a PMMA/PLA blendthat exhibits shape memory. PMMA/PLA blends having between 15-85% w/wPMMA and having between 15-85% PLA may be used to produce a PMMA/PLAblend that exhibits shape memory properties. The shape memory effect andthe moldability of a PMMA/PLA blend may be increased by adding anelastomeric additive, such as an impact modifier. The elastomericadditive, such as an impact modifier, may be added at a concentration of0 to 25% w/w of the total weight of the mixture. A temperature range of80 to 85° C. may be used in the heating step to form the polymer blendaround a fixture to give it a permanent shape. Alternatively, thepolymer blend may be heated and formed to a user’s face.

Many materials that are used for producing surgical masks experiencedeformation of the polymer fibers when heated near their thermoformabletemperature. Therefore, many thermoformed masks today have a layer ofmaterial that is thermoformable to provide the thermoformability of theproduct. This inherently results in a multi-layer structure, as opposedto the single layer structure as described herein.

A material that is a moldable thermoform, such as PMMA/PLA, enables amask to be produced that forms a better seal with the user’s face,compared to today’s surgical masks. A better fit could be offered toconsumers (mask wearers) by offering different sizes and shapes ofmasks. For example, a face mask that can be thermoformed withoutaltering or deforming the polymer fibers could be molded into small,medium, large, etc. sizes and could be offered in different shapes, suchas circular, oval, square, rectangular, etc. to better fit users faces.Custom surgical masks could also be produced by making a mold of anindividual’s face, fitting the moldable thermoform mask over the mold,and heating the mask material on the mold to give the mask a permanentshape.

It has been discovered that the PLA/PMMA compositions are particularlyuseful to form thermoformed fabric shapes such as thermoformed maskswithout deforming the fibers. Typically, a temperature of about 50 toabout 100° C. may be used to thermoform a shaped fabric such as a filtercomprising PMMA and PLA. A heated (and softened) mask can be pressed toa user’s face and as cools it forms to the contours of the user’s face.The thermoformed mask results in a face mask having a tight seal to theuser’s face. At least in part, because the fibers do not deform whenthermoforming, the shape may be altered and refitted or shaped foranother use or the like (reverse thermoformability). Other polymers orpolymer blends that exhibit shape-memory and that are capable offiltering may also be used in the disclosed embodiments.

A thermoformable filter material, such as a PMMA/PLA blend describedherein, may further include a backing layer to provide strength andsupport to the filter material. The backing layer may be a polymericmaterial that includes an adhesive on one side to adhere to the filtermaterial.

FIG. 2 shows a protective face mask 1100 comprising a thermoformablefiltering material 1101, a support 1102, and a support strap 1103. Thethermoformable filtering material 1101 may comprise a blend of PMMA andPLA. The thermoformable filtering material 1101 may comprise a blend of30% w/w PMMA and 70% w/w PLA. The thermoformable material 1101 maycomprise a blend of 15 to 85% w/w PMMA and 15 to 85% w/w PLA. Thethermoformable material 1101 may be produced via melt blown extrusionand may further include an elastomeric additive, such as an impactmodifier. The thermoformable material 1101 may exhibit shape memory whenheated, shaped, and cooled.

Support 1102 provides structure to the protective face mask 1100 andassists in holding the mask to a user’s face. Support 1102 may be apolymer that comprises a rubber elastomer. Support 1102 may be overmolded onto the thermoformable material 1101. Alternatively, support1102 may be a functional sealant that adheres to the thermoformablematerial 1101 and provides rigidity to the mask when cured. Support 1102may be a functional sealant that comprises or consists of silicone.

Support strap 1103 may be an elastic strap and functions to hold thesupport 1102 and the thermoformable material 1101 to a user’s face.Alternatively, support strap 1103 may be made of a non-elastomericmaterial. Support strap(s) 1103 may be slid through apertures(holes/openings) 1104 that are present in the support 1102, as shown inFIG. 2 .

FIGS. 3 through 9 show scanning electron microscope (SEM) micrographs ofa melt blown PMMA/PLA fabric useful for a filter. As shown in FIGS. 3through 9 , the fibers in the PMMA/PLA blend maintain their individualshape and are not deformed. This contrasts with, for example, somepolypropylene fabrics that are used in mask applications. In thesefabrics (see FIG. 10 , which is an SEM micrograph of a commerciallyavailable polypropylene mask produced by melt blowing), one can visuallyobserve fibers (using SEM) that are deformed. The deformation in thesepolypropylene mask fabrics may appear as parallel fibers melting intoone another to form a larger fiber. It may also appear as an individualfiber branching off along the fiber’s backbone (i.e., fray along thefiber length). The deformation may also appear as globules of pooledpolymer.

EXAMPLES

Nonwoven fabric filters are made by melt blowing through a die having a200 micrometer diameter orifice. The composition is 70% by weight PLA(INGEO Biopolymer 625F) and 30% by weight PMMA (PLASKOLITE CA41), unlessotherwise noted in Table 1. The melt extruder temperature is from about230 to 250° C., the die temperature was set at 200° C. and the blowingair set temperature is 380° C. and air pressure is 40 pounds per squareinch. The collector distance is as noted in Table 1. The filters weretested with salt particles in a similar manner as described inUS20060079145 (para. 28) except that the salt size was 0.1 and 0.2micrometer as shown in Table 1 and as described herein. The ComparativeExamples are made in the same way except that a charge enhancingadditive was added or the fabric was made with polypropylene as alsonoted in Table 1.

From Table 1, it is readily apparent that the filters of the inventionwhen charged have a greater efficiency for a given pressure dropcompared to polypropylene typically used for such filters. (see Examples6 and 7 versus Comparative Examples 5 and 6). Likewise, it is quiteevident that the filters of the present invention have a much lowerpressure drop for heavier weight fabrics allowing for more robustbreathable filters. Surprisingly, the filters and polymeric compositionof the present invention do not require a charge enhancing additive liketypical polypropylene filters (see Comparative Examples 1-3). This maybe due to the additive deleteriously affecting the desiredmicrostructure.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Comp. Ex. 1 Comp. Ex.2 Comp. Ex. 3 Comp. Ex. 4 Comp. Ex. 5 Comp. Ex. 6 Material * * * * * * *# # # PP PP PP Filter # of layers 1 1 2 3 1 2 3 1 2 3 1 2 3 Layer Fabricweight (g/m2) 95 95 95 95 95 95 95 95 95 95 30 30 30 Applied Charge (kV)0 40 40 40 40 40 40 40 40 40 40 40 40 Die Collector distance (inches) 55 5 5 6 6 6 5 5 5 5 5 5 Efficiency (0.1 micron) 41.2% 83% 92.6% 97.7%81.7% 94.9% 98.3% 55.9% 73.7% 84.0% 84.7% 94.9% 98.9% Efficiency (0.2micron) 28.5 85.9% 95.4% 99.3% 85.9% 97.5% 99.5% 53.4% 73.7% 86.5% 88.0%97.6% 99.7% Delta P @ 10 cm/s (Pa) 54.8 53.3 77.7 118.3 39.6 90.9 135.724.2 50.1 73.78 68.2 133.2 206.9 *70% PLA/30% PMMA # 70% PLA/30% PMMA +1% Magnesium Stearate PP - Polypropylene (Exxon Achieve PP6936G2)

1. A method of producing a non-woven material comprising: a) providing afeed mixture comprising polymethyl methacrylate and polylactic acid to amelt blown extrusion device; b) liquefying the feed mixture in theextrusion device to form a liquified feed mixture; c) blowing theliquified feed mixture through at least one nozzle to form a spray; d)depositing the spray on a surface; e) reducing a temperature of thespray such that the spray solidifies on the surface to form a sheet ofnon-woven material comprised of fibers.
 2. The method of producing thenon-woven material of claim 1 further comprising: forming a filteringmask comprised of filtering layer comprised of the non-woven material.3. The method of claim 1, wherein the sheet of non-woven material isthermoformable without deforming the fibers of the non-woven material.4. The method of claim 1, wherein the sheet of non-woven material has afiltering efficiency greater than 98%.
 5. The method of claim 4, whereinthe sheet of non-woven material has a pressure drop that is less than orequal to two millimeters of mercury.
 6. The method of claim 1, furthercomprising passing the fabric through an electric field. 7–34.(canceled)
 35. A filter comprising at least one layer of a nonwovenfabric comprised of fibers of poly(methylmethacrylate) and polylacticacid that have a diameter of about 0.5 micrometer to 15 micrometers andaverage diameter of about 2 to 8 micrometers.
 36. The filter of claim35, wherein at least about 90% by number of the fibers are longer than500 micrometers.
 37. The filter of claim 36, wherein the nonwoven fabrichas a fabric weight of 50 to 300 grams/m².
 38. The filter of claim 37,wherein the nonowoven fabric has a fabric weight of about 100 to 200grams/m².
 39. The filter of claim 35, wherein the filter has a pressuredrop of at most about 5 mm Hg.
 40. The filter of claim 35, wherein thefilter has a filter efficiency of at least 95%.
 41. The filter of claim40, wherein the filter efficiency is at least 98%.
 42. The filter ofclaim 35, wherein the filter has a surface potential of at least about50 volts.
 43. The filter of claim 42, wherein the surface potentialdecays at most about 5% after exposure to 70° C. for one hour.
 44. Thefilter of claim 43, wherein the efficiency is at least 95%. 45–49.(canceled)